netdev - Re: [PATCH v2] netfilter: properly initialize xt_table

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Date:   Fri, 18 May 2018 11:27:56 +0200
From:   Florian Westphal <fw@...len.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jan Engelhardt <jengelh@...i.de>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        Michal Kubecek <mkubecek@...e.cz>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] netfilter: properly initialize xt_table_info structure

Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> On Thu, May 17, 2018 at 12:42:00PM +0200, Jan Engelhardt wrote:
> > 
> > On Thursday 2018-05-17 12:09, Greg Kroah-Hartman wrote:
> > >> > --- a/net/netfilter/x_tables.c
> > >> > +++ b/net/netfilter/x_tables.c
> > >> > @@ -1183,11 +1183,10 @@ struct xt_table_info *xt_alloc_table_info(unsigned int size)
> > >> >  	 * than shoot all processes down before realizing there is nothing
> > >> >  	 * more to reclaim.
> > >> >  	 */
> > >> > -	info = kvmalloc(sz, GFP_KERNEL | __GFP_NORETRY);
> > >> > +	info = kvzalloc(sz, GFP_KERNEL | __GFP_NORETRY);
> > >> >  	if (!info)
> > >> >  		return NULL;
> > >>
> > >> I am curious, what particular path does not later overwrite the whole zone ?
> > >
> > >In do_ipt_get_ctl, the IPT_SO_GET_ENTRIES: option uses a len value that
> > >can be larger than the size of the structure itself.
> > >
> > >Then the data is copied to userspace in copy_entries_to_user() for ipv4
> > >and v6, and that's where the "bad data"
> > 
> > If the kernel incorrectly copies more bytes than it should, isn't that
> > a sign that may be going going past the end of the info buffer?
> > (And thus, zeroing won't truly fix the issue)
> 
> No, the buffer size is correct, we just aren't filling up the whole
> buffer as the data requested is smaller than the buffer size.

I have no objections to the patch but I'd like to understand what
problem its fixing.

Normal pattern is:
newinfo = xt_alloc_table_info(tmp.size);
copy_from_user(newinfo->entries, user + sizeof(tmp), tmp.size);

So inital value of the rule blob area should not matter.

Furthermore, when copying the rule blob back to userspace,
the kernel is not supposed to copy any padding back to userspace either,
since commit f32815d21d4d8287336fb9cef4d2d9e0866214c2 only the
user-relevant parts should be copied (some matches and targets allocate
kernel-private data such as pointers, and we did use to leak such pointer
values back to userspace).